The new Design Museum, formerly the Commonwealth Institute, on Kensington High Street in London.
Photograph: Alamy

A former head of English Heritage has warned that the country’s modernist legacy is being betrayed by a wrong-headed approach to 20th-century architecture.

Simon Thurley, who stepped down as chief executive in May 2015 after a 13-year stint, says landmark buildings are uniquely vulnerable to demolition, in part because modernist architecture tends to be unpopular with the public and politicians.

The problem is compounded by an unimaginative approach by conservationists. “At the moment, there’s a ‘one-size-fits-all’ philosophy of conservation,” Thurley told the Observer, “which is based on [keeping] the original fabric. For 20th-century buildings, the whole listed-building system, the legislation and everything based around keeping the fabric, is not relevant.

“These buildings are about ideas and other things. For the 20th century, you’ve got to have a different system. People don’t like 20th-century buildings. Politicians also think that, once a building’s listed, you can’t do anything about it. There’s a huge disincentive among the political classes to save them.”

The loss of some of the most innovative buildings of the postwar period would create a black hole in architectural history, he added. “The danger is that everything will be just got rid of and the next generation will have absolutely no idea about that extraordinary period of incredible optimism and determination to use architecture to transform society. We’ve got the whole thing muddled up … Everybody gets furious about it, but it’s not being handled properly. Saving the late 20th century is the most controversial and difficult area of modern conservation practice and debate.”

Modernist architecture, with its brutalist concrete forms and experimental designs, has inspired extremes of emotion. For example, Sir Denys Lasdun’s concrete National Theatre, on the South Bank in London, has been applauded as an icon of postwar architecture and mocked for resembling a nuclear power station. When such buildings are listed or receive public money for conservation, there is a storm of protest from those who see them as “ghastly”, Thurley said.

The solution, he argues, lies in radical revamps to improve them for contemporary use: “You need to justify greater intervention than you might in other types of historic building.”

Speaking after delivering the first of a series of public lectures on the theme at Gresham College, London, he said: “Having left English Heritage, I decided that I could politely say all the things I was bursting to say for 10 years about what’s wrong with the system.”

Under the current system, changes to listed buildings are dependent on local conservation officers and Historic England (formerly English Heritage). “If their attitude is that historic fabric must be saved at all cost, you’re not going to get that permission. They’ve got to adopt a new attitude, which is to protect the overall idea and significance.”

He cited the case of the Commonwealth Institute, the formerly derelict 1960s building in London, as an example of how listed modernist architecture can be improved: “When that building was being sold by the Commonwealth Institute, the trustees tried to get an act of parliament to have it delisted … It was all because the conservationists had been far too precious about what to do with these buildings. In 2007 it was sold to a development consortium who knew what they were doing.”

This month the building reopens as the new Design Museum. Thurley said: “Here’s an example of a very contentious case, which has come to a successful conclusion. If they make drastic interventions in 20th-century buildings, it gives them new life. I bet you, when people go into it, they will love it.”

He added: “You have got to put different standards on these 20th-century buildings. You cannot treat them like 18th-century houses. If you are too precious with them, you can actually prevent them from getting new uses. Some people say that the Design Museum conversion has destroyed this wonderful building and should never have been allowed. If it hadn’t been allowed, we wouldn’t still have the building.”

By contrast, Thurley criticised the demolition of the Grade II-listed Southside Halls of Residence, built in the 1960s for Imperial College London and admired for its fusion of “Oxbridge planning with great slab blocks of reinforced concrete”. He said the decision by English Heritage to support demolition when the building could have been repaired for a total of £40m, was extraordinary. “This is a problem faced by hundreds of highly graded buildings each year – the fact that the repair costs are greater than the eventual value of a building.

“But what about a Grade I-listed medieval church in the middle of nowhere? It has a £1m repair bill. Even if permission could be granted for its conversion to a house, it would only be worth £100,000 at most. So do we say, ‘Well, let’s demolish it’? No, of course we don’t.”

A similar fate is in prospect for Robin Hood Gardens, a public housing scheme in Poplar, east London, with large blocks arranged with deck access. The estate has been described as the embodiment of British brutalism but, in 2008, a listing application was rejected because there were other listed schemes of higher quality. “The building still stands but, with its certificate of immunity from listing, will soon be demolished,” Thurley said.

He acknowledged that “many of the most interesting and innovative” buildings have suffered because they were built quickly and cheaply, and some pushed technical capabilities of materials to their limits, such as the cladding on the Metropolitan Cathedral of Christ the King in Liverpool. Commenting on the perceived ugliness of so many 20th-century buildings, he recalled becoming director of the Museum of London in the 1990s and dismissing its then 1970s home as “a white-tiled lavatory built on a roundabout”. Over time, he said, he began to appreciate it.

“The problem is that [modernist architecture] does take greater understanding – almost philosophical understanding – to get it than historic architecture, which immediately appeals to the aesthetic eye.”